Toxic substances in the internal combustion engine, such as a diesel engine, are discharged as PM (particulate matter: soot composed of carbon fine particles, high-molecular weight hydrocarbon fine particles, sulfur-based fine particles such as sulfate, etc.). Since the Environmental Agency recently has considered deciding an environmental criterion with respect to fine particulates with particle size of 2.5 μm or less to more strictly regulate particulates, a development race, for example, of systems for controlling exhaust gas for a diesel engine or for PM monitoring, has been performed to respond to the stricter regulations.
As an exhaust emission control system for diesel engine, a sealed type ceramic honeycomb body (diesel PM filter: DPF) is prevalent. The DPF has a ceramic honeycomb structure in which both ends of opening parts of cells are alternately sealed. That is, the DPF includes inflow-side cells sealed on exhaust gas downstream side, outflow-side cells sealed on exhaust gas upstream side, and a cell partition wall defining the inflow-side cells and the outflow-side cells, respectively. The particulates are trapped by passing exhaust gas through pores of the cell partition wall.
However, since accumulation of the particulates causes increase in pressure loss of exhaust gas in the DPF, the DPF must be regenerated by removing the accumulated particulates to suppress deterioration of output or fuel consumption in the internal combustion engine. Therefore, forced regeneration of the DPF is being performed by burning the accumulated PM, for example, according to the following process. That is, the temperature of exhaust gas is raised by adding a reducing agent, such as fuel, to the exhaust gas, the reducing agent is burned with an oxidation catalyst disposed on the upstream side of the DPF, and the resulting high-temperature exhaust gas is then supplied to the DPF.
However, when such a regeneration control is performed in a state where the particulates are trapped within the filter in an accumulation amount beyond a certain filter use limit value, cracking or melting loss of the filter is caused by localization of temperature or excessive rise of overall temperature of the filter resulting from the burning of PM. For preventing such a failure, prediction of accumulation amount of particulates within the filter is performed by measuring the pressure loss in the filter, an intake air quantity, an exhaust gas temperature, a fuel injection quantity, an EGR opening, an engine rate or the like and performing arithmetic processing thereto in ECU.
On the other hand, in a common internal combustion engine such as the diesel engine, a value is obtained by multiplying a safety ratio to this filter use limit value and then adopted as a regeneration control point. The point is generally represented by:
Regeneration control point (g/L)=filter use limit value (g/L)×safe factor, wherein the safe factor is 0<safe factor<1. Accordingly, the regeneration control point is set so as to satisfy the relationship of:Regeneration control point (g/L)<filter use limit value (g/L).
This safety factor is differently set by each automobile maker, and is determined according to the completion rate of prediction technique for accumulation amount of particulates in the filter or the guideline for safety of each maker. As the safety factor is closer to 1, the filter regeneration becomes less frequent, so that the fuel efficiency is less deteriorated. Therefore, the accumulation amount of particulates in the filter needs to be accurately predicted.
In Japanese Patent Publication No. 2009-2276A, a transmitting antenna and a receiving antenna are mounted on a center part of an outer wall of a DPF filter so as to be opposed to each other. An electromagnetic wave of several tens GHz to several tens THz is transmitted from the transmitting antenna, passed through the filter, and received by the receiving antenna mounted on the opposite side. The accumulation amount of particulates to the filter is thereby calculated based on the receiving intensity of the electromagnetic wave. The electromagnetic wave is irradiated through an outer side wall of the filter in the direction perpendicular to the trapping holes of the filter.
Further according to Japanese Patent Publication No. 2009-250062A, electromagnetic wave is irradiated from an outer side wall of the filter in the direction perpendicular to the trapping holes of the filter.
According to Japanese Patent Publication No. 2009-57948A, electromagnetic wave is irradiated through an outer side wall of the filter in the direction perpendicular to the trapping holes of the filter. Further, the electromagnetic wave is irradiated to a plurality of positions on the side wall of the filter to detect the outgoing electromagnetic waves from a plurality of positions of the filter responsive to those, so that distribution of accumulation amounts in the filter is measured.
According to WO 2008/093729A1, it is described that electromagnetic wave is irradiated to a mesh made of a conductor while the optical axis of the electromagnetic wave is slightly inclined with respect to the mash plane to improve the received intensity of the electromagnetic wave.
Besides, Japanese Patent Publication No. 2007-79466A discloses a device of generating electromagnetic wave having thin plate structure.